2012-11-25

I've been a longtime user and sometimes fanboy of Racket, but aside from a few bug reports, before this week I hadn't contributed anything back to the language. This week, I started using a little helper macro, which wasn't in the core utils, to make some of my testing easier. I mentioned it to the super-friendly Racket community, they told me they liked it, and my pull request was merged within about 12 hours.

I've been using Racket for a while, so I knew roughly where to look to put my code, tests, and documentation. A newer user might not know, so this post outlines, in some detail, the steps I went through to put together a tiny feature extension for Racket.

A Tiny Feature

I'm dabbling in the implementation of a small scripting language called Pyret to study features of scripting objects. The language has a parser, which generates AST nodes. The nodes keep track of their location in the original program for error reporting, unbound identifier reporting, and the like. I wanted to write some test cases for our
parser, which generates things like:

Which checks that all the things I care about for the parse are true: the program parses to a block of code, with a single statement, which is a dot expression of the identifier o and the symbol x. With a little help from Jonah Kagan, I produced a macro that does exactly that, and works nicely with RackUnit, Racket's unit-testing framework (see it in action, with a slightly different name).

I thought check-match was pretty useful, and figured I'd see if the Racket folks at large would agree. I wrote a message to the Racket mailing list, figuring someone might think it was neat. There was some immediatepositivefeedback, so I decided to go ahead and try to add it.

Getting and Extending Racket

Racket's repo is hosted on Github. The easiest way to contribute is to fork it, and then check out your own copy. The check-out and build process is fairly standard; you should, however, make a directory called build/ to hold the binaries that will be created:

This takes about 20-30 minutes, and installs all the necessary Racket binaries locally in place (no sudo or anything needed).

Next up was to find RackUnit and the code I'd need to extend.

Most of what goes on in Racket's core utilities happens in collections, found in the collects/ directory of the base directory of the checkout. For my implementation, I'd be looking at collects/rackunit.

I want to implement a new kind of check, so let's find that in RackUnit. Here's what the RackUnit directory looks like:

But before I go sticking my code in there willy-nilly, it's important to realize there are three things that need to go with a commit like this:

Tests

Implementation

Documentation

We'll build up our commit in those stages.

Adding Tests

First, I need to know how I'm going to test this to make sure I don't screw anything up with my edits. There's actually a whole collection for tests in collects/tests/, which includes a rackunit subdirectory. Conveniently, this has been further divided into files that correspond to the files from the RackUnit collection itself:

So, we can add a few expected uses to check-test.rkt, which will be tested against the implementation. I found the end of the check-tests, and inserted some simple test cases there, using the existing style of the file:

Implementation and Running Tests

With the tests written, it's safe to go back and add my implementation to check.rkt, since I'll know if I've succeeded or not via these tests. I added my implementation there, with some comment caveats about how check-match differs from other checks:

In reality, this gives lousy error reporting, so the actual implementation leverages the helpful with-check-info form to populate the test with reporting information for failures.
With the implementation in place, it's time to run the tests, and figure out if what I did broke anything. To run a particular test suite, Racket provides a tool called raco that was built by the make install above. To run our tests, we do (from the base racket/ directory):

$ ./bin/raco test collects/tests/rackunit

I iterated through this a few times to suss out all the minor bugs in what I'd written. I also wanted to check that my tests were actually adding to the count, so I compared to the version without my changes by doing:

Keeping with the theme, we'll be editing check.scrbl which is the file that's used to generate this section of the RackUnit documentation.
Reading over the existing docs, I notice that our new feature is violating one of the principles of the existing documentation:

Although checks are implemented as macros, which is necessary to grab source location, they are conceptually functions. This means, for instance, checks always evaluate their arguments.

Based on Robby's recommendation (the mailing list is helpful and responsive again!) I simply added a caveat "(with the exception of @racket[check-match] below)", and moved on to adding actual documentation for check-match.
Scribble does two very cool things when documenting definitions. First, it has explicit syntax for telling the documentation system that you're introducing a new identifier that should be indexed and linkable. Second, it lets you write Racket code examples directly into the documentation, and even runs them and renders their results inline into the documenation. Here's a snippet of what I add:

@defform tells Scribble that this is a new syntactic form that should be indexed. Scribble figures out the the name is check-match, and adds links for it to the table of contents and enters it in the search index.

@racket[v] tells Scribble to render v as Racket code, and Scribble is also smart enough to know that v is the same v in the definition, and creates a back link for it.

@interaction[#:eval rackunit-eval ... ] blocks indicate expressions that should be run, with their output rendered after them. This makes for beautiful docs with examples inline to show users exactly what their getting.

To build the docs, we run:

$ ./bin/raco setup collects/rackunit

Then, the docs will appear in the local documentation directory. I can then open them up in a web browser and see the results (note the local url ending api.html; that's the local path to the documentation that's been installed):

Looks good!

Letting Racketeers Know

I packaged everything up in a single commit, and sent the whole thing off to the Racket folks with a pull request. They then reviewed it and incorporated it into their HEAD the next day.
The Racket folks maintain a list of Intro Projects, so there's easy places to start if you want to follow this tutorial and get involved!